The present invention relates to a pattern position detecting method for detecting the center position of a two-dimensional pattern and an apparatus for detecting the same and, more particularly, to a pattern position detecting method and an apparatus suited to the detection of a pattern for alignment when the reduced projection exposure of a circuit pattern onto a wafer are performed by reduction projection exposure equipment.
Generally, in reduced projection exposure equipment, a circuit pattern 4 on a reticle 1 and a wafer 3 on a wafer stage 7 are disposed with certain distance to allow a reduction projection lens 2 to be arranged therebetween as shown in FIG. 11. A condenser lens 18 is also provided at the upper position of the circuit pattern 4 on the, reticle 1. Exposure light from an exposure light source (not shown) is used to irradiate the circuit pattern 4 through the condenser lens 18 so that the reduction projection, exposure and duplication of the circuit pattern 4 is sequentially repeated for chips 51, 52, 53, etc. through an incident pupil 19 of said lens 2. To this end, the exact positioning of the circuit pattern 4 with each of the chips 51 to 53 is indispensable. This positioning, for example, of the chip 51 is performed using the so-called TTL (through-the-lens) alignment technique in which wafer target patterns 91 and 92 formed on the wafer 3 in advance and reticule reference patterns 81 and 82 (window patterns) formed on the reticle 1 beforehand are correctly positioned through the reduction projection lens 2.
However, in the conventional reduced projection exposure equipment 12, wafer-target-pattern-illumination light 10 is incident onto the center of the incident pupil 19 of the reduced projection lens 2 through a half-mirror 11 and the reticle reference pattern (window pattern) 81 to irradiate the wafer target pattern 91. Reflected light from the pattern 91 is again magnified and image-formed on the reticle reference pattern 81 through the reduction projection lens 2. Said patterns 81 and 91 are projected on the movable slit 13 by the magnification lens 12a. By the scanning of the movable slit 13, one dimensional signal 10a is output to a pre-processing circuit 16 from a photomultiplier 15 through a relay lens 14 and sent to a computer 17 after being subjected to the analog-to-digital (A/D) conversion at the circuit 16. Each center position of the reticle reference pattern 81 and the wafer target pattern 91 is obtained at the computer 17 to calculate the amount of alignment due to the difference between the center positions of those patterns 81 and 91. The wafer stage 7 is drive-controlled toward the x-direction depending on the alignment amount. The drive control of the stage 7 in the y-direction can be performed by the method similar to that of the x-direction drive control using the reticle reference pattern 82 and the wafer target pattern 92 specially provided for this purpose. For details of conventional equipment of this kind, reference can be made to the Japanese Patent Publication Nos. 144270/1978 and 99374/1979, for instance.
However, the following disadvantages can be pointed out in such conventional alignment equipment.
Since the reduction projection lens is generally designed to provide the best image formation for monochromatic light such as g line, it is necessary to use light having a narrow spectral width to be as close to monochromatic light as possible or laser light as the wafer-target-pattern illumination light.
However, the use of the light having a narrow spectral width causes a phase difference between light 20a reflecting and diffracting on a pattern 31 and light 20b reflecting and diffracting within the wafer target pattern 91 when the wafer-target-pattern-illumination light 10 is incident onto a resist 32 as shown in FIGS. 2(a) and 2(b). As a result, multiple interference is caused due to these reflected diffraction lights 20a and 20b having a different phase. Particularly, multiple interference fringes of narrow width appear in the usual pattern edge portion in which the film thickness of the resist 32 changes abruptly. The above-mentioned intensity of multiple interference consequently varies slightly to produce noise in said one dimensional signal 10a output to the pre-processing circuit 16 from the photomultiplier 15 as shown in FIG. 2(c). The relationship between said intensity of multiple interference and the film thickness of the resist 32 is shown in FIG. 3. In the drawing, a curve indicative of said intensity is shifted toward the left-hand and right-hand directions in response to an incident angle of 0.degree. at which light is given perpendicularly to the resist 32 (indicated at a solid line) and an incident angle of 20.degree. onto the resist 32 (indicated at a broken line).
Also, FIG. 4(a) is an enlarged perspective view of the state in which the wafer-target-pattern-illumination light 10 incident onto the center of said incident pupil 19 irradiates the wafer target pattern 91 in FIG. 1. As shown in FIG. 4(a), although the light 10 is given to the pupil 19 with the uniform quantity of light, an area 102 in which the incidence is made at an inclined angle .alpha. of 20.degree. (reduction projection lens NA=0.38) is larger than an area 101 corresponding to the perpendicular incidence (.alpha.=0.degree.). In other words, since the area 102 has energy greater than that of the area 101, the relationship between the film thickness of the resist 32 and the intensity of multiple interference assumes the state as indicated by the broken line in FIG. 3. On the other hand, most of reflected diffraction light 21 of said illumination light 10 with respect to light 103 from the area 101 is given to the incident pupil 19 of the reduction projection lens 2 as shown in FIG. 4(b). However, about one-ha1f of high frequency components (indicated at oblique lines) of reflected diffraction light 22 from the target pattern 91 with respect to light 104 from the area 102 for the inclined incidence cannot reach the incident pupil 19 by the disturbance of an outer frame 2a of said lens 2. The resultant detection signal 10a of the wafer target pattern 91 is shown in FIG. 4(e). As shown in FIG. 4(e), the pattern edge portion is not sharp as compared with that of FIG. 4(d). As a result, the contrast of the detection signal 10a is lowered to degrade the accuracy of alignment.